If you're like us, you blame most of your boob issues, such as pain or changes in size, on your period. And it's true, that can be why your boobs feel sore or look bigger. "Many changes are due to fluctuating hormone levels," says Richard Bleicher, M.D., surgical oncologist and director of the Breast Fellowship Program at Fox Chase Cancer Center in Philadelphia. But different stages of your menstrual cycle can affect your breasts in different ways, and other factors—diet, medications, and certain activities—can influence your dynamic duo too. In fact, while they may not be able to predict the weather à la Mean Girls' Karen Smith, your breasts tell you a whole lot about what's happening in your body and your health.

1. Exercise

In general, you're unlikely to notice any changes in your breasts even with an intense weightlifting regimen. That's because your breasts sit on top of the pec muscle, but aren't part of them. So you can develop stronger muscles underneath your breasts without affecting their size or shape, says Melissa Crosby, M.D., associate professor of plastic surgery at the University of Texas MD Anderson Cancer Center. However, a study from researchers in England who examined breast movement during exercise found that breasts bounce as much as 8 inches up and down during physical activity. All that movement can lead to breast pain post-sweat, but the researchers also found that wearing a sports bra reduces bounce by up to 78 percent during aerobic exercise. (These tips can help, too.) Want to learn more? Visit Shock Absorber. Enter your cup size and level of activity to watch what happens to breasts when you're on the move.

2. The Follicular Phase

Your menstrual cycle is divided into two halves: your follicular phase, during the first part of your cycle (your period is day one) and your luteal phase, after ovulation. During the follicular phase, especially five to seven days after your period, estrogen and progesterone levels are low and your breasts are at their minimum volume, says Jennifer Litton, M.D., associate processor of breast medical oncology at the University of Texas MD Anderson Cancer Center. Since they're the least hormonally stimulated during this time, this is the most accurate picture of what your breasts are really like.

3. The Luteal Phase

As estrogen and progesterone levels rise following ovulation, you experience an increase in blood flow to your breasts, which can increase their fullness, nodularity, and tenderness. In a study of more than 200 premenopausal women who received mammograms in both the follicular and luteal phase, researchers found that breast density and size was greater in the luteal phase. "If you feel something unusual in your breast, track it during different times in your menstrual cycle because it may be due to changes in your hormones," Dr. Litton says. If it doesn't go away, have your doctor check it out.

4. Changes in Weight

Your breasts are made up of breast tissue (including lobules and ducts that are called into action while breastfeeding) and fat tissue. So when you gain weight, your breasts increase in size. When you lose weight, you may notice they shrink. The amount of fat each woman gains or loses in her breasts depends on breast composition, which isn't the same for everyone. Some women have denser breasts, which means they have more breast tissue and less fatty tissue. (Weight isn't the only thing that impacts breast type.) These women may not notice as large a shift in breast size when they gain and lose weight as a woman who has a greater proportion of fatty tissue would.

5. Bites

"I don't know why, but spiders seem to love breasts," says Mary Jane Minkin, M.D., professor of obstetrics and gynecology at Yale School of Medicine. (Ed note: OMFG.) "It can be really scary because you may develop a red, inflamed bump and women often think they have inflammatory breast cancer, when it's just a reaction to a spider bite that can be cleared up with antibiotics." It's unlikely that inflammatory breast cancer (a rare, but very aggressive form of the disease) develops overnight, so if you suddenly notice redness, inflammation, and swelling, it could be a bug bite. But still be sure to have you doctor look at it, Dr. Minkin adds.

6. Alcohol

Women often underestimate the impact booze can have on their breast health, Dr. Minkin says. More than 100 studies have looked at the association between alcohol consumption and breast cancer risk in women. A recent U.K. study of more than 28,000 ladies found that even low levels of alcohol consumption—less than one drink per day—increased the risk of breast cancer by about 12 percent. Scary stuff! It's not exactly known how booze boosts your risk, but it could affect levels of hormones, such as estrogen, associated with tumor growth. Drinking may also lead to weight gain and your risk of breast cancer rises with your weight. "Stick to no more than one drink per day," Dr. Minkin says.

7. Pregnancy

The majority of changes that occur in your breasts during pregnancy prepare your body for breastfeeding. Blood vessels, ducts, and lobules in your breasts that are involved in producing and transporting milk expand and proliferate. This process can lead to tenderness, heaviness, and a significant increase in your cup size. You'll also notice your areola and nipples expand.

8. Breastfeeding

Soon after delivery, you experience a surge in the hormone prolactin that tells the mammary glands in your breasts to produce milk. When you're breastfeeding, many different things can stimulate prolactin and milk letdown such as a baby crying (even if it's not yours), talking about or touching your baby, and sex. If you're worried about leaking milk during sex, wear a bra with some absorbent nursing pads. "Even when you finish breastfeeding, your breasts will continue to express milk for at least another six months. so don't be alarmed if you still squirt out some milk with a little stimulation," says Dr. Minkin.

9. Sex

Your guy shouldn't be the only one ogling your breasts during sex—check them out next time you're getting frisky. During foreplay, your heart rate and blood pressure increase, which causes your breasts to swell and your veins to become more prominent. Your nipples become erect (yep, he's not the only one sporting an erection!) and the areolae—the area around the nipple—expand. A Rutgers University study even identified a link between areas of the brain that respond to nipple stimulation and those connected with clitoral stimulation. This connection may explain why some women can orgasm from nipple stimulation alone (you lucky minx)!

10. Menopause

"Breasts are hormonally sensitive tissue so you'd think that they would become smaller after menopause because estrogen levels are lower, but that's not actually what happens," Dr. Minkin says. On average, women pack on over a pound per year after menopause, according to a study in Annals of Behavioral Medicine. (This is likely due to a decrease in physical activity and lean muscle mass, and shifts in hormones that cause you to store more fat in your midsection.) Unfortunately, packing on 20 pounds or more after menopause is associated with an 18 percent higher risk of breast cancer compared to women who gain little or no weight, according to a study in JAMA. As you age, you may also notice that your breasts become softer and may start to sag. Breast density decreases with age so you have a greater proportion of fatty tissue than you did when you were younger. Plus, Cooper's ligaments—fibrous tissue that acts like an internal bra—stretch over time and may lead to drooping. Regularly sporting a supportive bra can help.

11. Birth Control Pills

"When birth control pills were first invented, they contained extremely high doses of estrogen and there were a lot of women walking around with very large breasts as a result," Dr. Minkin says. (We're looking at you, Joan Holloway.) Today, most contain about one-fifth of the original amount, so they don't have as big an effect. Still, it's completely normal if you experience a little boost in breast size when you start birth control. "Estrogen can lead to more fluid retention and also increase breast tenderness," Dr. Minkin adds. If it's troublesome, talk to your doctor about other options.

12. Medications

Besides the Pill, certain other meds—especially some types of antipsychotic and antidepressant drugs—are notorious for affecting your breasts. One in particular, Risperdal, is a drug used to treat bipolar disorder. Risperdal can increase prolactin levels, which may cause your breasts to leak milk even if you're not breastfeeding, Dr. Minkin says. For women who experience breast pain around their period, nonsteroidal anti-inflammatory drugs (such as ibuprofen and aspirin) can help. They may work by inhibiting prostglandins that are involved in inflammation and tenderness, Dr. Bleicher says. Diuretics, often used to treat high blood pressure, can also decrease breast pain and swelling by reducing fluid retention, Dr. Minkin adds. While some minor pain and discomfort is considered normal at certain times during your cycle, talk to your doctor about issues that impact your lifestyle.

13. Caffeine

Sad, but true: Your morning cup (or three) can take a toll on your breast health. Some women's breast tissue is especially sensitive to caffeine. She may experience fibrocystic breasts—a benign condition that can make your ta-tas feel lumpy or rope-like. For some, this can be incredibly painful and worsen around your period. Doctors don't know why this occurs, but if you experience breast pain or "nodularity," as doctors refer to the lumpiness, try cutting out caffeine for several weeks, Dr. Minkin says. (Consider trying one of these seven caffeine-free drinks for energy instead.) If you experience severe soreness and stemming your coffee intake doesn't help, talk to your doctor.

14. Implants

This one goes without saying, right? You get breast implants, you're probably not Googling, "Why are my boobs bigger?" What you may not realize, though, is that although breast implants are often meant to make your breasts appear larger, they can cause some of your natural breast tissue to atrophy. This may be due to the implant pressing against your breast tissue, causing the tissue to break down over time. (Oh, and FYI: With breast implants, you can still receive mammograms—but clinical breast exams may be more challenging. "You're mostly feeling the implant, so it's difficult to identify abnormalities in the natural breast tissue," Dr. Minkin says. If you have implants, discuss with your doctor the best approach for screenings.)

15. Puberty

Another estrogen-fueled change. At first a breast bud—a small raised bump under your nipple—starts to grow. Next, the nipple and areola grow larger and darken in color and over time breasts continue to grow. They don't always sprout at the same pace—one may be larger than the other for a long time or even forever. It can take several years for your breasts to reach their full size. What else to look forward to when this process starts? Your first period. On average, it starts about two to two-and-a-half years after breast development begins.